This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-215319, filed Jul. 29, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a nonaqueous electrolyte secondary battery.
Presently, a thin lithium ion secondary battery is put on the market as a nonaqueous electrolyte secondary battery for portable apparatuses such as a portable telephone. The thin lithium ion secondary battery comprises a positive electrode containing lithium cobalt oxide (LiCoO2), a negative electrode containing a graphitized material or a carbonized material, a separator interposed between the positive electrode and the negative electrode and consisting of a porous membrane, a liquid nonaqueous electrolyte prepared by dissolving a lithium salt in an organic solvent, and a jacket consisting of a cylindrical or rectangular can.
With progress in miniaturization and thinning of the portable apparatus, it is required that the secondary battery be made thinner and lighter in weight. However, it is somewhat difficult to realize a secondary battery of the construction described above having a thickness not larger than 4 mm.
Under the circumstances, proposed is a nonaqueous electrolyte secondary battery comprising an electrode group including a positive electrode, a negative electrode, and a polymer electrolyte layer interposed between the positive electrode and the negative electrode, and a jacket consisting of a laminate film having a thickness not larger than 0.25 mm, and the particular nonaqueous electrolyte secondary battery is being vigorously developed. In the secondary battery provided with the polymer electrolyte layer, the bonding strength between the electrode and the electrolyte layer can be ensured even if the thickness of the jacket is decreased.
On the other hand, various ideas are being proposed in an attempt to decrease the thickness of a nonaqueous electrolyte secondary battery provided with a liquid nonaqueous electrolyte. For example, a lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator having mutually facing surfaces holding a liquid electrolyte, and an adhesive resin layer consisting of a high molecular weight gel phase and a high molecular weight solid phase containing a liquid electrolyte and a liquid electrolyte and the adhesive resin layer serving to permit the positive electrode and the negative electrode to be bonded to the mutually facing surfaces of the separator is proposed in Japanese Patent Disclosure (Kokai) No. 10-177865.
On the other hand, Japanese Patent Disclosure No. 10-189054 discloses a method of manufacturing a liquid ion secondary battery, comprising the steps of preparing a positive electrode by forming a positive electrode active material layer on a positive electrode current collector and a negative electrode by forming a negative electrode active material layer on a negative electrode current collector; preparing a solution of a binder resin consisting essentially of polyvinylidene fluoride by dissolving polyvinylidene fluoride in a solvent and coating a separator with the binder resin solution; preparing a battery laminate body by arranging the positive electrode on one surface of the separator and the negative electrode on the other surface of the separator, followed by drying the resultant laminate structure so as to evaporate the solvent; and impregnating the battery laminate body with a liquid electrolyte.
It is also proposed in Japanese Patent Disclosure No. 10-172606 that an adhesive resin layer is arranged between a positive electrode prepared by bonding a positive electrode active material layer to a current collector and a separator and between a negative electrode prepared by bonding a negative electrode active material layer to a current collector and the separator. It is taught that, in this case, the bonding strength between the positive electrode active material layer and the separator is made equal to or higher than the bonding strength between the positive electrode active material layer and the current collector. It is also taught that the bonding strength between the negative electrode active material layer and the separator is made equal to or higher than the bonding strength between the negative electrode active material layer and the current collector.
In each of the lithium ion secondary batteries disclosed in the prior arts described above, it is possible to ensure a sufficiently high bonding strength between the positive electrode and the separator and between the negative electrode and the separator, even if the thickness of the jacket is decreased. In addition, since it is possible to use a liquid nonaqueous electrolyte, it is possible to increase the volume energy density and the large current discharge characteristics, compared with the secondary battery utilizing a polymer electrolyte.
Incidentally, in an attempt to further increase the capacity of the nonaqueous electrolyte secondary battery, it is being studied to use lithium nickel complex oxide (LiXNiO2) in place of lithium cobalt complex oxide (LiXCoO2) that was widely used in the past as a positive electrode active material. For example, Japanese Patent Disclosure No. 63-121258 discloses lithium nickel complex compounds having a heterogeneous element such as Al, Sn, In, B, P, or Si introduced therein. Also, a lithium nickel cobalt complex oxide (LixNi1xe2x88x92yCoyO2) is disclosed in J. Power Sources, 43-44, 595 (1993). Further, it is described in J. Electrochem. Soc., 142, 4033 (1995) that a lithium nickel oxide having aluminum introduced therein exhibits a relatively high thermal stability.
However, the nonaqueous electrolyte secondary battery comprising a positive electrode containing the lithium nickel complex oxide or the lithium nickel cobalt complex oxide described above as an active material and a jacket having a wall thickness not larger than 0.25 mm, which certainly permits improving the capacity, is inferior in its safety.
To be more specific, where the jacket has a wall thickness not larger than 0.25 mm, the phenomenon such as gas generation or temperature elevation within the secondary battery tends to bring about an accident such as deformation of the battery, gas spurting or ignition, compared with the case where the wall thickness exceeds 0.25 mm. Therefore, in the secondary battery comprising a jacket having a wall thickness not larger than 0.25 mm, it is necessary to suppress the gas generation and temperature elevation within the battery as much as possible. However, since the lithium nickel complex oxide and the lithium nickel cobalt complex oxide noted above are inferior to the lithium cobalt complex oxide in the thermal stability, an oxygen gas is generated if the temperature within the battery is rapidly elevated to 80xc2x0 C. to 100xc2x0 C. because of, for example, short-circuiting within the battery. The oxygen gas thus generated reacts with the organic solvent contained in the nonaqueous electrolyte so as to bring about an oxidizing decomposition of the nonaqueous electrolyte. As a result, the battery temperature is further elevated so as to cause a gas to be spurted from within the battery or to cause a danger of ignition.
An object of the present invention is to improve both the capacity and safety of a nonaqueous electrolyte secondary battery comprising a jacket made of a thin sheet material.
According to a first aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a jacket having a wall thickness not larger than 0.25 mm, a positive electrode housed in the jacket and containing a positive electrode active material, a negative electrode housed in the jacket and containing a negative electrode active material, and a nonaqueous electrolyte housed in the jacket,
wherein the positive electrode active material comprises at least one kind of oxide selected from the group consisting of an oxide containing an element M, Li and Ni and an oxide containing an element M, Li, Ni and Co, the element M being at least one element selected from the group consisting of Al, B, Sn and Nb, and the pH of the positive electrode active material falls within a range of between 10 and 12.
According to a second aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a jacket having a wall thickness not larger than 0.25 mm, a positive electrode housed in the jacket and containing a positive electrode active material, a negative electrode housed in the jacket and containing a negative electrode active material, and a nonaqueous electrolyte housed in the jacket and containing a nonaqueous solvent and a solute dissolved in the nonaqueous solvent,
wherein the positive electrode active material comprises at least one kind of oxide selected from the group consisting of an oxide containing an element M, Li and Ni and an oxide containing an element M, Li, Ni and Co, the element M being at least one element selected from the group consisting of Al, B, Sn and Nb, and the nonaqueous solvent contains at least 50% by volume of xcex3-butyrolactone.
According to a third aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a jacket having a wall thickness not larger than 0.25 mm, a positive electrode housed in the jacket and containing a positive electrode active material, a negative electrode housed in the jacket and containing at least one kind of carbon material selected from the group consisting of a fibrous carbon material, a spherical carbon material and a granular carbon material and a nonaqueous electrolyte housed in the jacket and containing a nonaqueous solvent and a solute dissolved in the nonaqueous solvent,
Wherein the positive electrode active material comprises at least one kind of oxide selected from the group consisting of an oxide containing an element M, Li and Ni and an oxide containing an element M, Li, Ni and Co, the element M being at least one element M selected from the group consisting of Al, B, Sn and Nb, and the nonaqueous solvent containing at least 50% by volume of xcex3-butyrolactone.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.